Muscle contraction

What happens to the length of a muscle when it contracts?

shortens

aka Concentric contractions, it shortens towards the center and is determined by the strength of the pulling force. It can go both ways, by generating force that increases action, or can be relaxed and shortened by another mover.

actin and myosin filaments

Sliding Filament Mechanism

Is how the sacromeres shorten, the actin and the myosin slide along each other. A message is sent from the nervous system to tell it to contract, the message causes the sarcoplasmic reticulum to release stored calcium into the sacroplasm. These calcium ions attach onto the actin.